The present invention concerns a mechanical, low-noise press, and particularly but not exclusively a screw press of the fly or friction type.
Known types of presses comprise a ram bearing a striking mass or tup, driven by a sturdy screw with a practically vertical axis, which engages a fixed nut thread which is integrally mounted on the machine's vertical posts.
The displacement of the tup is obtained by rotating the screw, generally by means of a central wheel keyed on its axis, such wheel being alternatively brought into contact with two lateral wheels, which are driven in opposite directions by a motor, so that one of the wheels controls the upward movement, the other one the downward movement of the tup.
These presses, as well as others, are usually rather noisy, not so much because of the impact of the tup on the piece being formed or on the die, but rather because of the impact and collision between the screw and the nut thread during the recovery of their mating backlash, following the action of the tup during the working phase.
In fact, during the collision of the tup against the piece to be formed, its kinetic energy is transformed, in part, into deformation and heat transferred to the piece and, in part, it is transmitted to the screw and the nut thread, thereby generating very high friction and overheating between the contacting surfaces, with subsequent mechanical and acoustic vibrations of very high intensity.
The noise level is further enhanced by the fact that, in general, the screw is made of high-resistance steel and the nut-thread is made of bronze, which materials give rise to different resonance phenomena induced by collisions and vibrations. Heretofore, many attempts have been made to reduce the noise level of such presses, mostly striving to acoustically insulate the structure of the machines from the rooms containing them, with rather unsatisfying results and at fairly high expense.
As is known in all the machines of this type a centralized, automatic, normal lubrication system is provided which obviously involves also the screw and nut-thread coupling. However, the lubrication practically has no influence on the noise and the acoustic behavior of the assembly.
In German patent application DE-A-2 851 551, a screw press is described which comprises a lubrication and cooling device for the screw/nut thread assembly, having the purpose of solving the problem of overheating of the two components during the working phase of the press. One attempt to solve the overheating problem is made by means of ducts arranged near the meshing threads, wherein a cooling medium, in a liquid or gas form, is forced through the ducts. The medium is collected into a supply tank and brought into circulation by means of a pump or by gravity, after it has been cooled in a suitable external heat-exchanger. In a particular embodiment, the liquid medium is oil which is collected in a chamber formed between the bottom walls of the screw and the nut thread, which together form a piston/cylinder assembly.
Although this device improves the working conditions of the press and permits a reduction in the clearances between the screw and the nut thread, it is not without inconveniences since it leaves practically unchanged the contact conditions between the two main components and thus it does not help to decrease the noise level of the assembly.